A conventional heat exchanger is known which is disclosed in, for example, JP-A-4-45393. That is, the heat exchanger employs a loop heat pipe including a flow rate adjustment valve disposed in a midway point of a condensation duct from a condenser to an evaporator and adapted to adjust the flow rate of a heat transfer fluid flowing therein.
A branch duct is provided to be branched and connected with a midway point of an evaporation duct between the evaporator and the condenser. The branch duct is provided with a driving portion that is adapted to be operated by a vapor pressure of a predetermined value or more received from the evaporation duct. The flow rate adjustment valve has an emergency closing valve provided on a condensation side for closing the condensation duct in cooperation with the operation of the driving portion.
Thus, the degree of opening of the flow rate adjustment valve is controlled according to a temperature of fluid to which heat is transferred in the condenser, so that an amount of heat transferred to the fluid is adjusted, while the pressure in a heat pump is kept below a set value.
When the flow rate adjustment valve constantly remains opened due to any abnormal condition, the excessive evaporation continues at the evaporator. When a vapor pressure exceeds a predetermined value, the driving portion is driven to operate the emergency closing valve in cooperation with the driving portion, thereby closing the condensation duct. Thus, the flow of the heat transfer fluid is forcefully stopped, thereby preventing an abnormal increase in vapor pressure up to above the predetermined value, and also preventing the breakdown or the like of the heat pipe.
Although the heat-pipe heat exchanger prevents the increase in vapor pressure due to the excessive evaporation at the evaporator by use of the emergency closing valve, the heat exchanger cannot prevent an excessive increase in temperature of the heat-transferred fluid at the condenser.
For example, when the supply of heat from a heating source to the evaporator is abruptly stopped and at the same time the flowing of the heat-transferred fluid is stopped at the condenser, the heat-transferred fluid may have its temperature increasing excessively without closing the emergency closing valve while the evaporation effect continues due to the remaining heat from the heating source.